The present invention relates generally to joining articles, such as heat exchanger components, by brazing. More particularly, the present invention relates to inventive methods of brazing and adhering brazing materials to components to be brazed. The invention also includes coating compositions for use in such methods.
One popular brazing process is the NOKOLOC brazing process developed by Alcan International Ltd., of Montreal, Canada. Briefly, the process includes the steps of forming an assembly, applying a brazing slurry to the assembly, blowing off the excess slurry, drying the slurry onto the assembly, and then passing the assembly through a brazing furnace. The brazing slurry generally includes a brazing alloy and a flux material, but may include additional materials.
The NOKOLOC brazing process is commonly used for the manufacture of all- aluminum heat transfer products such as condensers and oil coolers used for automotive and commercial applications. Corrosion of such components is an important issue in the industry, especially in automotive applications due to the constant exposure to road salts, acid raid and the like. Consequently, it has become a common practice in the industry to flame spray small amounts of zinc onto the components to protect the surfaces of the tubes and fins of such heat transfer products. However, the flame spraying process is expensive, wasteful and presents safety and environmental risks. In addition, this process has led to difficulties in the brazing process and in the resultant product. For example, uneven or heavy application of the zinc can erode the aluminum tubes and can cause holes to form in the final product. In addition, the zinc becomes an important part of the "filler metal" between the components since the zinc melts at a much lower temperature than the brazing material. This results in a "zinc rich filet" between the components, and such filets have a tendency to corrode rather rapidly because the zinc is used as a "sacrificial" material that corrodes before the underlying components.
Another problem encountered in the NOKOLOC brazing process is that more "filler metal" is often required at the junction between the tubes and headers of a heat exchanger assembly than between the tubes and fins. To overcome this problem, many manufacturers currently apply a paste-like substance which contains a high amount of powdered silicon to act as a filler metal in the junction between the tubes and headers prior to brazing. The paste-like substance provides additional filler metal between the tubes and headers and adds additional mechanical strength to the filet. One such substance is known under the designation Omni-Paste and is commercially available from Omni-Technologies Corp. of Epping, N.H. However, such materials are expensive and occasionally interfere with the brazing process through the action of silicon erosion of the underlying aluminum tubes and headers.
One other known method for providing more "filler metal" at the juncture between the tubes and headers is disclosed in U.S. Pat. No. 5,100,048. That patent discloses a process of applying pure silicon powder to the heat exchanger components, and the powder, at brazing temperatures, diffuses into the underlying aluminum material and causes an eutectic mixture to form. The eutectic mixture then melts at a lower temperature and forms a filler metal that brazes the assembly together. However, it has been found that the pure silicon powder does not work well when added to the brazing slurry in the NOKOLOC brazing process. Rather, the silicon powder must be applied to the tubes in a more evenly distributed manner. When the silicon is not evenly applied to the components, the silicon causes uneven formation of the filler metal, which leads to erosion of the tubes, fins and headers of the product.
One advantageous method of brazing heat exchanger components in order to overcome the foregoing deficiencies in the prior art is described in co-invented and co-owned U.S. Pat. No. 5,544,698 (U.S. application Ser. No. 08/220,159). Briefly, that patent teaches advantageous methods of applying differential coating compositions of brazing materials to heat exchanger components prior to assembling and brazing the components. By pre-coating the aluminum or other material with selected differential coatings, the invention permits relatively uniform and selective application of zinc and silicon (as well as other brazing materials) to the aluminum components to avoid problems associated with the prior art brazing processes. In one embodiment, the patent teaches that a liquid cement can be used to apply the brazing materials (such as powdered or particular flux material, brazing alloy, zinc or silicon) onto the aluminum components in a coating. One such liquid cement, which is both a binder and vehicle, is commercially available and sold under the trademark NICROBRAZ, and available from Wall Colmonoy Corporation, Madison Heights, Mi. However, such liquid cements are toxic and result in environmental, safety and health concerns when used in a brazing process. Such liquid cements may also have adverse chemical reactions with other brazing materials and may leave a corrosive residue on the assembly.
Adhesives other than liquid cements can also present certain problems when used in the brazing process. Specifically, such adhesives are almost universally hydrocarbons, and the heat of the brazing process can break the bonds between the hydrogen and carbon atoms, leaving unwanted carbon residue on the finished product. The unwanted carbon residue can cause corrosion initiation sites to form on the heat exchanger components.